This invention relates to the conversion of natural gas to syngas, and then to liquid hydrocarbons that are easily transported.
Substantial proportions of known natural gas reserves are situated in locations remote from areas of high consumption. There is about 3.5 TCF (100 billion cubic meters) of natural gas flared annually worldwide. Nearly 40% is in Africa, and ˜17%, ˜12%, and ˜17% in North America, Central and South America, and the Far East & Oceania respectively. The amount of natural gas being flared is estimated to be equivalent to 900,000 barrels per day (bpd) of liquid product. Conversion of both stranded gas and associated gas into a transportable and saleable form of products is a major challenge and at the same time represents enormous business potential.
Natural gas can be liquefied and transported to larger users. For example, Liquefied Natural Gas (LNG) accounts for 97% of Japan's natural gas supply. LNG accounts for about 90% of the outlet for stranded gas, with methanol at 7% and Fischer-Tropsch/Gas to Liquid (F-T/GTL) less than 2%. LNG, however, is capital-intensive and is typically only economically viable for large gas reserves located at coastal sites. The large investment required and the small number of receiving terminals limit the marketing flexibility of LNG.
An alternative to LNG is the gas to liquid (GTL) route that converts natural gas into synthetic fuels ranging from gasoline to middle distillates, as well as to methanol and other liquids. This approach avoids the infrastructure limitations associated with LNG and at the same time provides a market that is large enough to accept the potentially large volumes of product. Synthetic liquids and other synthetic petroleum products are clean and cheaper to transport, market, and distribute to large markets than LNG. They can be transported in existing pipelines or product tankers and even blended with existing crude oil pools. Furthermore, no special contractual arrangements are required for their sale, and such fuels are not subjected to OPEC regulations. GTL products offer an advantage to conventional fuels in that the low sulfur content of the GTL fuels leads to significant reductions in particulate matter that is generated during combustion. Their low aromatic content reduces the toxicity of the particulate matter. There is a worldwide trend towards the reduction of sulfur and aromatics in fuel. These factors are major drivers behind the GTL process development and investment.
U.S. Pat. Nos. 6,596,781 and 6,495,610 describe processes that produce syngas that is used in more than one type of GTL process. However, these processes suffer from one or more of the problems described below.
The scale-up exponent (cost is a constant times capacity y) of GTL plants is estimated to be 0.66. In the absence of a breakthrough technology, therefore, economies of scale are the only significant mechanism by which GTL can achieve economic viability. At present large fields will have LNG and F-T processes followed by other natural gas consumers whereas smaller fields will accommodate large methanol units producing 10,000 tons per day (TPD) methanol which will flood the methanol market.
To prevent flooding the market with a single GTL product, it is desirable to be able to convert a large amount of natural gas into syngas, and then distribute the syngas to a variety of GTL processes. Depending on market demand and other factors, different types of products can be produced at different rates from a large-scale syngas generation hub. Some of the products that may typically be produced are middle distillate/diesel, gasoline, methanol (MeOH), dimethyl ether (DME), lubricants, or other liquid hydrocarbons. Using a large-scale syngas hub also allows the use of moderate size DME units without the normal investment penalty for smaller scale plants. However, distributing the syngas to various processes suffers from the following problems:                1) The requirement for syngas composition (e.g., H2/CO) by different liquid products is quite different. Thus, the same syngas composition cannot be fed to a variety of GTL processes.        2) The range of syngas composition that can be produced by a single synthesis unit (such as an autothermal reforming plant (ATR)) is limited by factors such as steam/carbon ratio, etc.        
Thus, ATR units are typically designed to produce a relatively narrow range H2/CO ratio syngas. For example, if the ATR is producing syngas to be fed to an F-T unit, the unit will be designed to produce a syngas with a H2/CO ratio of about 1.9.
To address the limitations imposed by syngas unit designs, it is desirable to develop a process wherein the H2/CO ratio of the syngas can be adjusted after leaving the syngas production unit without wasting a significant amount of the gas produced. Specifically, it is desirable to adjust the H2/CO and (H2−CO2)/(CO+CO2) ratio, to meet specifications required by different liquid production processes that are integrated with the syngas production unit to convert the syngas into easily transportable liquid hydrocarbon products.